Out of those there should be 5*60 + 40 = 340 minutes of talks to be given in one thread, thus having 2200 minutes (37 hours) in parallel.

As we'd like to start not earlier than 10 a.m. and wrap up at 6 p.m., minus 1 hour lunch, minus about an hour for coffee breaks and breaks between talks we come to 6 hours of talks a day, minus 1.5 hour of those given in one thread (keynotes and lightning talks). Not counting an auction.

Thus we can have 4.5*3 = 13.5 hours of parallel talks during the three main days, which is 40.5 hours if we have three rooms, and 54 hours if we have four.

If every submitted talk is accepted, we already are able to fill three talk rooms for the whole conference.

In any words, as we have four rooms, we can still accept about 14 hours of talks without renting yet another room.